As well known in rotary piston engines, an eccentric shaft with rotor journal portions having axes offset to the axis of the eccentric shaft is disposed longitudinally extended in the center of the rotary piston for mounting eccentrically rotating rotors. Each rotor defines three cavities in the working chamber formed by the rotor housing having an inner wall of trochoidal configuration and a pair of side housings attached to the end surface of the rotor housing abutting the working chamber.
In the multiple-rotor type rotary piston engine having an eccentric shaft with rotor journal portions corresponding to each rotor, if the journals of the eccentric shaft are provided between the rotor journal portions, the installation of the eccentric shaft into the engine housing is difficult. Generally speaking, this difficulty relates to the geometrical relation between larger rotor journal portions and smaller eccentric journal portions. In case that the eccentric journal portion is as large as the rotor journal portion, though the installation may be facilitated, other serious problems, such as, escape of compressed gas through the eccentric journal portion are encountered. Therefore, the eccentric journal portions are formed smaller than the rotor journal portions.
In order to resolve the problem, an eccentric shaft assembly composed of divided eccentric shafts each of which corresponds to one of the rotors has been proposed in the prior art, for example, in Japanese Utility Model (Jikkosho) 45-8482. As proposed, the eccentric shaft assembly is made for the four-rotor type piston engine. These divided eccentric shafts are connected in meshing engagement with each other, and are integrated by one through bolt. Such an eccentric shaft assembly, however, has a weak point in tightness of connection because the bolt may be loosen by heat deformations or stress deformations. If the bolt loosens, then unallowable vibration of the engine will be experienced. Also it is difficult to connect four divided eccentric shafts precisely on the same common axis in view of production tolerances and errors.
On the other hand, Japanese Laid-open Patent Application No. (Tokkaisho) 60-69208 shows another eccentric shaft structure having no through bolt. The eccentric shaft assembly of this publication is designed especially for a three-rotor type piston engine, and therefore, is not capable for use in a four-rotor type piston engine. Even if one were attempt to add another rotor eccentric portion by way of an auxiliary eccentric shaft joined with a main eccentric shaft so that the auxiliary eccentric shaft and the main eccentric shaft have two rotor eccentric portions, respectively, the eccentric shaft assembly would have unallowable vibration and one would have difficulty in centering of the shafts.